GRUB

(GRand Unified Boot Loader) :If you have multiple kernel images installed on your system, you can choose which one to be executed.
“GRUB just loads and executes Kernel and initrd images.”
initial RAM disk (initrd) is a temporary root file system that is mounted during system boot to support the two-state boot process. The initrd contains various executables and drivers that permit the real root file system to be mounted, after which the initrd RAM disk is unmounted and its memory freed.
The initrd contains a minimal set of directories and executables to achieve this, such as the insmod tool to install kernel modules into the kernel.

Kernel:

Mounts the root fle system “Executes the /sbin/init program”Init: locks at the /etc/inittab file to decide the lnux run level